For modern lighting means, such as light emitting diodes (LEDs) it is necessary to control the current through such lighting means for adjusting characteristics of the emitted light. In particular, LEDs require a direct current and therefore measures are necessary to operate LEDs in a regular electric power system. Such electric power system provides an alternating voltage that needs to be rectified before it can be supplied to the LED. But rectification alone is not sufficient to operate the LED with the desired output. Thus, after rectification of the alternating voltage, the average current which is supplied to the LED needs to be controlled. By doing so, the emitted spectra and light intensity can be controlled.
Different proposals have been made in order to provide such controlled current. In order to adjust the current through the LED it is known to use switched converters, as for example described in WO 2011/076898. Here, the provision of electrical power to the LED in particular for dimming the LED is described. The average current that is supplied to the LED is dependent on the switch-on time of the switched isolated converter. Since in such isolated converters no direct measurement of the current through the LED is possible, measured values of the primary side of the converter are used to determine the switch-on time and thus in combination with the rectified input voltage, determine the effective current through the LED. But the known solutions for determining the switch-on time of the switched converter are based on the assumption that the elements that are used in the circuit have ideal characteristics. Of course, real elements do not have such ideal characteristics but show parasitic capacitances for example. Such a parasitic capacitance has a negative effect on the measurement, because the process of switching after the switch of the converter is switched off is slower than in an ideal case. As a consequence, the current that is present on the secondary side and thus flows to the LED is larger than expected. More than that, the influence of the parasitic capacitance is dependent also on the load and changes when the LED is dimmed for example. As a consequence the control circuit is not able to adjust the desired current on the output side correctly.
Thus, it is an object of the present invention to provide an improved driving circuit and respective method for providing an operating current to at least one lighting means.